Fat-Free Mass Index Calculator: What the Number Measures

Fat-free mass index (FFMI) relates a person’s estimated fat-free mass to height. A common formula is fat-free mass in kilograms divided by height in metres squared. For instance, if fat-free mass is 60 kg and height is 1.70 m, the calculation is 60 ÷ (1.70 × 1.70). The example demonstrates the arithmetic only and is not a target or health judgment.

Fat-free mass is not just muscle

Fat-free mass includes body components other than fat, such as water, organs, and bone; it is not interchangeable with skeletal muscle mass. The calculator’s FFMI result depends on how fat-free mass was estimated. A consumer scale, skinfold method, or clinical body-composition assessment may use different assumptions and precision. If the underlying estimate changes, the index changes too.

Fat-free mass index explainer showing estimated fat-free mass divided by height squared, with a reminder that the measurement method matters. — Original explainer: FFMI depends on a fat-free-mass estimate, not a direct measure of muscle alone.

How to interpret FFMI

FFMI is a descriptive index, not a diagnosis of health, nutrition, athletic potential, or disease. A single value cannot account for medical history, age, or the method used to estimate body composition. Be cautious with charts that label a value as universally “good” or “bad” without explaining who the reference applies to and how the measurement was obtained.

The CDC notes that BMI does not distinguish fat, muscle, and bone. FFMI is designed around a different input, but it still depends on measurement and context. If you are concerned about muscle loss, weakness, nutrition, or an unexpected change, discuss it with a healthcare professional. Do not begin a restrictive diet or supplement routine based on an index alone.

Compare the concept with our muscle-mass percentage article and read the CDC explanation of BMI limitations.

Silent visual explainer. Transcript: FFMI divides estimated fat-free mass by height squared; it is not a direct muscle or health diagnosis.

Common questions

What is the FFMI formula?

Fat-free mass in kilograms divided by height in metres squared.

Is fat-free mass the same as muscle mass?

No. Fat-free mass includes more than skeletal muscle.

Can FFMI diagnose low muscle?

No. Discuss concerns with a professional who can assess your situation.

Sources

Educational information only; not a diagnosis or personal fitness prescription.

Fat Free Mass Index Calculator